The present invention relates to pulse width modulated rectifier systems and, more particularly, to an improved method and apparatus for control of such systems.
A conventional controlled rectifier for converting alternating current (AC) to direct current (DC) utilizes two pairs of serially connected switching devices with each pair connected in parallel between positive and negative voltage output busses. Reversely poled diodes bypass each switching device. AC power is coupled to the rectifier at junctions intermediate each pair of switching devices. The rectifier is operated in a pulse width modulation (PWM) mode which allows the output DC voltage to exceed the voltage which could be obtained using diodes alone. In this operational mode, pump-up inductors are coupled in circuit between the AC source and the rectifier. Switching devices are selectively gated into conduction such that the inductor is connected across the AC source to cause current to build up to some selected level. When the devices are switched off, the reactance of the inductor causes current to continue, which current is forced to flow to the DC output busses through the diodes.
There are some disadvantages to use of a PWM rectifier in the above described form. One disadvantage is that the switching devices must be operated at a higher frequency, e.g., such as at 500 Hz or higher in order to minimize losses due to harmonic currents. This higher frequency of operation increases losses in the switching devices. Another disadvantage is the generation of harmonics which can be introduced into the AC power system. This latter disadvantage is particularly noticeable when multiple, synchronized rectifiers are coupled to the AC power system such as may occur when the controlled rectifiers are used on transit cars to provide DC voltage which can be converted to controlled frequency AC power for driving AC electric traction motors.
During multiple transit car operation, the inductance and resistance of the catenary or third rail between cars is very small compared to the inductance of each transformer on board each transit car and the inductance of the pump-up reactor connected in series with the secondary of the transformer. Accordingly, the voltage at the transformer in each car will be substantially in phase and equal. Since the cars are connected to a common trainline so that the operator commands are the same for each car, the load represented by the electric traction motors on each car is substantially the same. Accordingly, each rectifier input voltage and its phase angle and the rectifier current and its phase angle are approximately the same for each car. If the angle difference between the voltage and current is relatively small, then the second harmonic will be shifted by twice the angle and the nth harmonic will be shifted by N times the angle if the control and modulation scheme are the same. When the number of transit cars and PWM rectifiers increases, the harmonic currents add linearly and may interfere with telephone and other communication signals along the transit car right of way.